1. Field of the Invention
The present invention relates generally to a riser system and, more particularly, to a riser system for coupling a subsea drilling or storage point to a floating vessel.
2. Description of Related Art
The advent of subsea oil and gas operations has posed a great number of technical difficulties that have been addressed with varying degrees of success. One recurring problem involves providing coupling lines between the subsea well head or template and the work or production platform on the surface of the sea. Such coupling lines are required for the passage of equipment to and from the subsea location and for the delivery of fluids to and from the subsea location both during the drilling operation and during later production or servicing. Due to the quantities of materials transported between the subsea location and the platform, these coupling lines, or "risers" as they are known in the art, are most often relatively large, heavy lines, and can be of considerable length. Because the risers are in effect the litelines of a subsea operation, their integrity and reliability are key to the success and profitability of the installation.
A principal difficulty associated with riser systems is bending due in part to the weight of the riser, but primarily resulting from movement of the surface platform or vessel relative to the seabed. The surface structure coupled to the subsea location via a riser generally has at least some degree of freedom of movement with respect to the subsea location. Such movement is most often caused by such factors as tides, currents, swells, wave action, wind, and thermal effects. Moreover, modern deep water installations make use of floating surface structures tied to the seabed by anchors and mooring lines at some distance from the subsea template. Thrusters may also be employed to maintain the position of the surface platform or vessel relative to the seabed. However, even where such anchoring and positioning techniques are used, floating structures still exhibit more movement than do fixed drilling and production platforms. Water depths in excess of about 300 meters require compliant floating platforms that, in turn, create the need for somewhat flexible riser conduit connections between the seabed and the platform due to the bending that occurs in the riser as the platform or vessel moves on the sea surface. In addition, even in relatively shallow water, heavy risers may undergo considerable bending stresses that may threaten the integrity of the riser system.
Risers are typically deployed either in straight sections descending directly from the surface platform to the seabed or in a continuous bend catenary configuration in which the riser follows a natural bend radius determined by its size, rigidity, and length. Straight risers generally employ flexible joints at the upper and/or lower ends of the riser. A disadvantage of such risers is the need for riser tensioner systems to support the weight of the column of pipe to prevent buckling. In the catenary configuration, the weight of the riser itself provides tensioning for the vertical, or near vertical, sections of pipe descending from the surface platform or vessel. As the riser approaches the seabed, it assumes a continuous bend due to its natural tendancy to droop under its own weight and under the loads imposed on it. The catenary configuration is particularly useful in greater depths of water where the surface platform or vessel is positioned at some distance from the subsea tie-in point. However, as larger sizes of pipe, having wider bending radii, are utilized for the riser, the bending stresses resulting from the catenary configuration increase. Thus, the size of risers that are capable of deployment in the catenary configuration is effectively limited by the bending radius and bending stresses.
To mitigate the effect of stresses on riser systems, buoyancy or ballasting elements may be attached to at least a portion of the submerged length of the riser. Such elements usually comprise synthetic foam elements or individual buoyancy or ballasting tanks. These tanks may be formed or deployed on the outer surface of riser sections and, unlike foam elements, are capable of being selectively ballasted with water or inflated with air using the floating vessel's air compression equipment. These buoyancy devices create upwardly directed forces in the riser and thereby compensate somewhat for stresses created by the weight of the riser. However, experience shows that such buoyancy devices cannot generally compensate for sufficient stress to preclude failure of the riser.
Several solutions to the problem of limiting the effect of bending stresses in riser systems are currently offered. One solution includes the use of flexible hose or conduit. Such flexible conduit may be employed with bend stiffeners, or stress joints, at the upper end where the riser ties into the platform, at the lower end where the riser ties into the well head or template, or at both the upper and lower ends. This solution may provide a riser system that is somewhat immune to the adverse structural effects of bending stresses because the conduit is relatively free to bend, thus transmitting little bending moment to the conduit wall. However, the use of flexible conduit has several drawbacks. First, flexible conduit is expensive. In addition, such conduit is more susceptible to damage than metal pipe due to its structure and the materials of which it is constructed. Finally, reliability and service life of flexible conduit in riser systems have yet to be proven. While bend stiffeners generally extend the service life of flexible conduit, they add significantly to the cost of the overall riser system.
Another known solution is the use of rigid pipe sections welded end to end to form a continuous riser. Although the term "rigid pipe" would seem to imply a lack of flexibility, in reality, even large, heavy pipe permits some bending without rupture; and over the total length of pipe utilized in a typical riser system, a considerable degree of bending is often possible. Rigid pipe risers are often equipped with flexible connectors at the upper and/or lower ends of the riser. Such flexible connectors function to tie the riser into the platform piping or the seabed template and, typically, accommodate several degrees of swivel of the riser pipe relative to the tie-in point. While this solution otters the advantage of being less costly than flexible conduit risers, the pipe comprising the riser is much more susceptible to bending stresses resulting from movement of the platform or vessel. In addition, as mentioned above, the size of the pipe employed in the riser is limited in the catenary configuration.
While the goal of minimizing the bending stresses in immersed pipelines has been generally recognized in the prior art, no satisfactory solution has yet been proposed that could be extended to application on riser systems. The present invention is directed to overcoming or minimizing the problems set forth above.